Our objective is to understand the defects in cellular immunity that lead to autoimmune insulin-dependent diabetes mellitus (IDDM). We believe that study of the BB rat will help us achieve this goal. We hypothesize that IDDM in the BB rat is the result of an imbalance between beta cytotoxic effector cells and regulatory T cells. This hypothesis is founded on three observations we have made previously. 1) Spleen cells from diabetic BB rats adoptively transfer the disease to naive recipients. 2) Transfusions of lymphocytes prevent IDDM in BB rats provided that cells expressing RT6 become engrafted. 3) In vivo depletion of RT6+ T cells induces diabetes in the resistant subline of BB rats. From the first observation we infer the existence of autoimmune effector cells. From the second and third, we infer the existence of RT6+ regulatory T cells that can prevent IDDM. We believe that the expression of IDDM depends on the balance between effector and regulatory cells. Understanding what these cells are, how they function, and how the equilibrium between them is regulated are the issues on which this proposal is focused. Specific Aim No. 1 focuses on the mechanism(s) by which regulatory RT6+ T cells prevent IDDM. We will determine the exact phenotype and secretory products of regulatory RT6+ T cells using newly developed in vivo assay systems. These investigations will be complemented by studies of the circulating form of the RT6 molecule as a marker of disease. We will also test the hypothesis that soluble regulatory factors prevent IDDM. These studies have been stimulated by our observation that RT6+ T cells sequestered in diffusion chambers prevent IDDM. Using both recombinant RT6.1 protein and transgenic animals expressing RT6, the immunoregulatory properties of the molecule will be tested. Specific Aim No. 2 is to investigate factors that render effector T cells diabetogenic or nondiabetogenic as a result of exposure to immunoregulatory activity. We will determine the phenotype and secretory products of effector RT6 T cells, focusing on their activation requirements in the presence and absence of regulation. Our analysis suggests that the relative balance between effector and regulatory T cells modulates the expression of IDDM, providing a paradigm for understanding autoimmunity in general. The results of our studies will define the T cell populations in BB rats that determine whether IDDM will occur. They will tell us how these populations interact and how the equilibrium between them is maintained or disrupted. We hope that an understanding of these cellular interactions will eventually translate into the design of rational strategies for the prevention of human juvenile diabetes.